Raster output scanners, such as a laser printer, capable of printing a complete page line-wise with serially presented data are well known. Typically, front-end conversion systems connected to a workstation for inputting text and graphical data are required for laser printers. The laser printer includes a light beam which is modulated image-wise, and the modulated light beam is deflected line-wise by means of a polygonal mirror over a light-sensitive surface. In a known manner a latent image can be written to the light-sensitive surface by means of the modulated light beam. The latent image is thereafter developed in a known manner and transferred to a receiving material such as a sheet of paper.
By means of the work station it is possible to input text or other information to be printed on a page. The input text (coded) is expanded in the front-end by font data stored in a memory and the expanded text is then stored in a page-size bit-map memory. The work station is normally provided with a display for composing the page to be printed. The layout of the page can be varied as required through the work station. However, because many kinds of fonts of variable sizes may have to be placed in the bit-map memory at predetermined positions, the time required to decode and place all of the characters in the memory is dependent upon the complexity of the page.
When a print order is given, for example to print a number of different pages consecutively, the front-end must be able to perform the decoding and placement within the available time. In a printer using a photosensitive layer containing a number of fixed imaging sections, charging the imaging sections must be accomplished at an early stage. However, in a raster output scanner operating continuously, before the imagewise exposure can be started, there is the undesirable possibility that the page to be printed is so complex that the front-end has not completed the filling of the bit-map memory by the time the charged imaging section has arrived at the imaging section. The charged photosensitive layer then has to be completely exposed by means of a light source to prevent the charged image part from being developed completely black. This undesirable charging and exposure ages the photosensitive layer more quickly than necessary.
Accordingly, it is an object of the present invention to provide a method and apparatus for limiting the undesireable exposures and thereby reduce the aging caused by such exposures.